Superluminal synthetic motion with an optical space-time metasurface

The speed of transfer of mass may not be greater than the speed of light, however, the apparent motion of objects may be superluminal. Space-time modulations of a surface that exhibit superluminal motion have been the topic of myriad theoretical investigations that promise nanoscale control over electromagnetic waves via the Doppler effect and Fresnel drag. Herein, we use the ultrafast photoexcitation of a 40 nm film of Indium Tin Oxide to generate superluminally travelling modulations, which act as diffractive apertures in both the spatial and temporal domains. By designing continuous and discrete forms of motion we realise a tuneable platform for generation of complex momentum-frequency beam profiles and exploring exotic phenomena like hawking radiation.